1. Field of the Invention
The present invention relates to a method of displaying the speed and distance on board a motor vehicle, generally implemented by means of an electronic device which avoids having a mechanical connection between the speed sensor with which one of the wheels of the vehicle is provided and the device displaying the speed and the distance covered, namely the speedometer and the mileage recorder with which the dashboard is provided. In general, the speedometer and mileage recorder are of conventional type, that is to say whose input member is a rotary shaft on which a small electric motor, or auxiliary motor, is installed controlled by the electronic device.
2. Description of the Prior Art
Such a method is already known in which
a first period of recurrence of a train of pulses controlled by rotation of one of the wheels of the vehicle is determined,
a second period of recurrence of a train of pulses is determined controlled by the rotation of the shaft of the auxiliary motor, connected to a device displaying the rotational speed of said shaft, and a totalizer totalling the number of revolutions of said shaft from putting the vehicle on the road,
the product of said first period multiplied by a factor depending on the development or size of the wheel is determined,
the difference between said second period and said product is determined,
the rotational speed of the auxiliary motor is controlled so that it is proportional to the preceding difference.
This method is put into practice by means of an electronic device which forms a loop slaving the second period to the product of a first period by the factor of multiplication dependent on the wheel development or size.
In other words, the rotational speed of the auxiliary motor is made dependent on the rotational speed of the wheel, the factor of multiplication apart.
It is known that real servo control loops are imperfect, in particular insofar as their response time is concerned. Thus, a very rapid variation of the rotational speed of the wheel will only be imperfectly recopied by a variation of the rotational speed of the auxiliary motor, at least during the first moments. The result is that the value of the speed displayed by the speed indicator during sudden acceleration or deceleration phases does not correspond to the true value of the speed. This forms not in itself a considerable disadvantage and may be tolerated, to the extent that, although it is important to know accurately the speed of the vehicle in the phases, where this speed is relatively stable, it is not very serious to have only approximate knowledge thereof when accelerating or braking.
However, a serious consequence of the existence of a time response of the servo control loop is that the distance recorder which totalizes the number of revolutions of the output shaft of the auxiliary motor, does not indicate a number of kilometers strictly equal to that really covered. Thus, since the auxiliary motor does not rotate fast enough in the sudden acceleration phases, the number of additional kilometers indicated during these phases is less than the number of additional kilometers actually covered; on the other hand, in the sudden deceleration phases, the auxiliary motor rotates too fast and the number of additional kilometers indicated is greater than the number of additional kilometers actually covered. Unfortunately, the accelerations and decelerations are such that there is no compensation and the total number of kilometers displayed by the mileage recorder is not equal to the total number of kilometers actually covered.